Quan el tren surt amb retard, la meva mare sempre busca un altre seient.

Here quan introduces a time clause. In a sentence like this, it can be understood as when or whenever, because the whole sentence describes a habitual situation.

So Quan el tren surt amb retard... means something like When/Whenever the train leaves late...

Catalan often uses the simple present this way for repeated events.

Catalan uses definite articles more often than English. El tren is the normal way to say the train.

Even when English might sometimes sound more general, Catalan still often keeps the article. So el tren is completely natural here.

Surt is the 3rd person singular form of sortir, which means to go out, to leave, or to depart.

So:

• jo surto = I leave / I go out
• tu surts = you leave
• ell / ella surt = he/she leaves
• el tren surt = the train leaves / departs

In this sentence, el tren surt means the train departs.

Amb retard literally means with delay, but in natural English it usually corresponds to late or delayed.

So surt amb retard means leaves late or departs behind schedule.

This is a common expression in Catalan:

• anar amb retard = to be delayed
• sortir amb retard = to leave late

Catalan uses the present tense for habitual or repeated actions, just like English does.

So:

• Quan el tren surt amb retard, la meva mare sempre busca...

means that this is something that usually happens, not just one specific event.

It is the same idea as English:

• When the train leaves late, my mother always looks for another seat.

The first part, Quan el tren surt amb retard, is a subordinate time clause placed before the main clause.

When this kind of clause comes first, Catalan normally uses a comma:

• Quan..., la meva mare...

This is very similar to English:

• When the train leaves late, my mother always...

If the order were reversed, the comma might disappear:

• La meva mare sempre busca un altre seient quan el tren surt amb retard.

In Catalan, possessives are usually used together with the definite article:

• la meva mare = my mother
• el meu pare = my father
• la seva casa = his/her house

So la meva mare is the normal structure. English does not use an article here, but Catalan usually does.

Adverbs like sempre often go before the main verb in Catalan:

• sempre busca
• sempre arriba tard
• sempre menja aquí

This placement is very natural. Catalan word order is a bit more flexible than English, but sempre busca is the most straightforward choice here.

Because the sentence describes a habit, not an action happening right now.

• busca = looks for / searches for
• està buscant = is looking for / is searching for

Here the meaning is general and repeated, so the simple present busca is the right choice.

Because seient is a singular countable noun, and Catalan normally needs an article or determiner with it.

So:

• un seient = a seat
• un altre seient = another seat

Using altre seient by itself would sound incomplete in standard Catalan.

Because seient is a masculine singular noun.

So the words agreeing with it are also masculine singular:

• un = masculine singular indefinite article
• altre = masculine singular form of other/another

Compare:

• un altre seient = another seat
• una altra cadira = another chair

So the form changes according to the gender of the noun.

Yes. Seient is the standard word for seat, especially in places like trains, buses, cars, or theaters.

So buscar un altre seient means to look for another seat.

You might also hear other words in specific contexts, but seient is the most direct and standard choice here.